1. Field of the Invention
The present invention relates to the production of nanoporous silica dielectric films and to semiconductor devices and integrated circuits comprising these improved films. The nanoporous films of the invention are prepared using silicon containing pre-polymers and are prepared by the use of double end-capped porogens to prevent chemical attachment of the porogen to the Si-network. As a result essentially all the available silanol (Si—OH) groups can be cross-linked to give a rigid network before the removal of the porogen, thus producing a nanoporous film with few silanol groups.
2. Description of the Related Art
As feature sizes in integrated circuits are reduced to below 0.15 μm and below, in new generations of electronic devices, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. One of the solutions to overcome these difficulties is to develop materials of dielectric constant less than about 2.5 for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials used in the manufacture of integrated circuits.
One type of material with a low dielectric constant is nanoporous silica films prepared from silicon containing pre-polymers by a spin-on sol-gel technique. Air has a dielectric constant of 1, and when air is introduced into a suitable silica material having a nanometer-scale pore structure, such films can be prepared with relatively low dielectric constants (“k”). Nanoporous silica materials are attractive because similar precursors, including organic-substituted silanes, such as tetraacetoxysilane (TAS)/methyltriacetoxysilane (MTAS)-derived silicon polymers are used as the base matrix and are used for the currently employed spin-on-glasses (“S.O.G.”) and chemical vapor deposition (“CVD”) of silica SiO2. Such materials have demonstrated high mechanical strength as indicated by modulus and stud pull data. Mechanical properties can be optimized by controlling the pore size distribution of the porous film. Nanoporous silica materials are attractive because it is possible to control the pore size, and hence the density, mechanical strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous films offer other advantages including thermal stability to 900° C.; substantially small pore size, i.e., at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit; preparation from materials such as silica and tetraethoxysilane (TEOS) that are widely used in semiconductors; the ability to “tune” the dielectric constant of nanoporous silica over a wide range; and deposition of a nanoporous film can be achieved using tools similar to those employed for conventional S.O.G. processing.
High porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. An additional advantage, is that additional compositions and processes may be employed to produce nanoporous films while varying the relative density of the material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous films that controls the dielectric constant of the material, and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the degree of porosity decreases, and vice versa. This suggests that the density range of nanoporous films must be optimally balanced between the desired range of low dielectric constant and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate suitable for the purpose. Usually, a precursor in the form of, e.g., a spin-on-glass composition is applied to a substrate, and then polymerized in such a way as to form a dielectric film comprising nanometer-scale voids.
When forming such nanoporous films, e.g., by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and water to cause polymerization/gelation (“aging”) during an initial heating step. In order to achieve maximum strength through pore size selection, a low molecular weight porogen is used.
U.S. Pat. No. 5,895,263 describes forming a nanoporous silica dielectric film on a substrate, e.g., a wafer, by applying a composition comprising decomposable polymer and organic polysilica i.e., including condensed or polymerized silicon polymer, heating the composition to further condense the polysilica, and decomposing the decomposable polymer to form a porous dielectric layer. This process, like many of the previously employed methods of forming nanoporous films on semiconductors, has the disadvantage of requiring heating for both the aging or condensing process, and for the removal of a polymer to form the nanoporous film. Furthermore, there is a disadvantage that organic polysilica, contained in a precursor solution, tends to increase in molecular weight after the solution is prepared; consequently, the viscosity of such precursor solutions increases during storage, and the thickness of films made from stored solutions will increase as the age of the solution increases. The instability of organic polysilica thus requires short shelf life, cold storage, and fine tuning of the coating parameters to achieve consistent film properties in a microelectronics/integrated circuit manufacturing process.
Formation of a stable porous structure relies on the condition that the porogen removal temperature is higher than the crosslinking temperature (or the gel temperature) of the matrix material. It was found that a stable nanoporous structure of less than 10 nm average pore size diameter cannot be produced when the concentration of the alkali cation such as sodium is below 200-300 parts per billion (ppb) level in the spin-on solution. However, stringent requirement for low metal concentration must be met for IC applications. The general practice is to have metal concentration below 50 ppb in the spin-on solution. Therefore, there is a need to develop a low metal nanoporous silica film that can consistently give dielectric constant less than 2.5 and average pore size diameter less than about 10 nm in diameter.
In the past, in order to achieve maximum strength through pore size selection, low molecular weight polyethylene glycol monomethyl ether was chosen as the porogen. Formation of a stable porous structure relies on the condition that the porogen removal temperature is higher than the cross-linking temperature (or the gel temperature) of the matrix material. It was observed that such porogen could chemically react with the Si-network, capping the free silanol groups that are involved in crossing linking reactions during the process. Such species decompose to give un-wanted isolated Si—OH groups after the final curing stage that is taken place at a much higher temperature. The changes in hydrophilicity resulting from silanol groups detrimentally impact the dielectric properties. Thus, in order to obtain dielectric materials with low and stable k values, it is desirable to minimize the amount of isolated silanol groups present in the final film. Further more, having free silanol groups also leads to un-desirable out-gassing in the IC integration. The stringent requirement for low out-gassing and stable k must be met for IC applications. The general practice is to obtain hydrophobic films. Therefore, there is a need to develop a hydrophobic nanoporous silica film that can consistently give dielectric constant less than 2.2 and absorb as little moisture as possible. Furthermore, it was widely assumed that pores are formed as the result of chemical attachment of porogens onto the silica network.
It has now been found that chemical attachment is not necessary to form porous silica. It has now been found that through the use of a double end-capped polyethylene oxide porogen, the formation of porous silica resulting from a physical blending of porogen and silicon pre-polymer gives a more hydrophobic film as suggested by its lower delta k value between films at ambient and after heating. The effect of double end-capped porogens, such as poly(ethylene glycol)dimethyl ether, is to prevent any chemical attachment of the porogen to the Si-network so that no additional silanol will be generated during the removal of the porogen and the existing silanol groups will be cross-linked to full extent possible, thus producing a nanoporous film with few, if any, silanol groups. Through the additional use of onium ions or nucleophiles the formation of a porous silica network at lower temperature in a low metal spin-on formulation can be facilitated. The effect of the onium ions or nucleophiles is to lower the gel temperature so that the rigid network is set in before the removal of the porogen, thus producing a nanoporous film without the presence of an alkali ion. The function of the porogen is to control the pore size and to readily decompose after the formation of stable pores. Other side-reactions that prevent the extent of the cross-linking of sol-gel reactions are minimized.